Diatom biostratigraphy of the middle Miocene marine sequence of the Kawadani section in the Kamogawa area, Boso Peninsula, central Japan

Abstract

Diatom biostratigraphy is established for the Middle Miocene Kinone and lower Amatsu formations in the Kawadani section, Boso Peninsula, central Japan. Nine out of 31 collected calcareous nodules yield age diagnostic diatom fossils. The Denticulopsis hyalina Zone (NPD 4B), Crucidenticula nicobarica Zone (NPD 5A) and Denticulopsis praedimorpha Zone (NPD 5B) are recognized from the upper Kinone Formation to lower Amatsu Formation. A diatom assemblage correlated with the Thalassiosira fraga Zone (NPD 2A) is found in a reworked nodule in the Kanigawa Formation. Useful biohorizons such as the fisrt occurrence (FO) of D. praedimorpha var. minor (D51), the last occurrence (LO) of C. nicobarica (D52), the first acme (AC1) of Denticulopsis crassa (D52.5) and the FO of D. praedimorpha var. praedimorpha (D53) are recognized in the lowest Amatsu Formation. Based on the stratigraphic position of these biohorizons, the stratigraphic relationship between biohorizons of microfossils in the Boso Peninsula is revised. Comparison of the stratigraphic relations of diatom and calcareous nannofossil biohorizons between the Boso Peninsula and northeastern Japan areas suggests a diachronism for the last occurrence of calcareous nannofossil Cyclicargolithus floridanus.